1. Field of the Invention
The present invention relates to a toner for developing static charge images, and more particularly to a toner for use in a system with low developing potential.
2. Description of Related Arts
Electrophotography generally refers to the formation of an electrical latent image on a photo-sensitive medium, which is developed by a toner and transferred, as necessary, to paper or other suitable media, and fixed under heat or pressure. The developing agents used for electrophotography fall into two general categories: two-component agents consisting of a toner and a carrier, and one-component agents provided with both toner and carrier functions.
The one-component agents are further subdivided into magnetic and non-magnetic types, the former containing 10% to 70% by weight of magnetic powder. The magnetic type is still further subdivided, in terms of developing driving force, into electrically conductive and insulating types, the former being driven by electrostatic induction or charge injection, and the latter by triboelectrification.
Development by one-component electrically-conductive (hereafter, "conductive" refers to electrical conductivity) magnetic toner yields uniform images having no edge effects, because the conductive magnetic toner itself works as the developing electrode. It is also known that such a toner type has the advantage of being applicable to a low-potential development system having a developing potential of 100 V or less, by controlling the volumetric resistivity of the toner at roughly below 1.times.10.sup.4 .OMEGA..multidot.cm.
However, the conductive magnetic toner has several disadvantages. The charges tend to leak via the transfer paper during the electrostatic transfer process, making it difficult to transfer the toner onto plain paper. Another disadvantage is the difficulty in securing the necessary image concentrations, as the toner particles are developed in only one layer on the photosensitive medium.
The above transfer-related problems are solved to some extent by using paper specially treated to have high resistivity or by employing a pressure transfer process in which a rubber roller is used. However, it is essential to secure image concentrations in the transfer process, which is not satisfactorily realized by the prior art.